The present disclosure relates generally to techniques for producing an optically variable film, an apparatus and a method for making the film.
Holographic Optical Elements (HOE) may be formed by creating diffraction gratings in a substrate. An early method to produce diffraction gratings utilized ruling engines that would scribe, line by line, with a diamond tipped stylus, the grooves to produce the diffraction grating. The scribing was typically done in metals and was very time consuming.
Another method of forming HOE involves creating interference patterns or gratings in a surface, which, when viewed, gives rise to holographic visual effects. One method of creating interference patterns involves selectively exposing regions of photosensitive material to interfering beams of illumination. The exposed regions of material become insoluble and the unexposed regions are dissolved away, leaving a desired pattern on the material. This method may be enhanced with e-beam exposures to reshape a sinusoidal structure creating blazed grating profiles.
The patterns formed by the method above may be treated as the final holographic image, or may be used as a master to form other equivalent surfaces, for example, by embossing with heat and pressure or UV cast embossing. The master may be copied by electroforming in nickel or itself vacuum metalized to strengthen its surface before embossing to make the separate mechanical impression or impressions.
Another process to create interference patterns includes forming a plurality of pixels in a surface so that the holographic image is formed by an aggregate of the pixels. In such a process, two or more light beams are directed toward a surface to interfere with one another at a desired pixel location, thereby forming the diffraction grating pixel. Pixels may be individually formed across the surface to create an array of pixelated diffraction gratings. Computer programs may control both the interfering directionality of the illumination and the locations of the individual pixels.
The methods above rely on polymerization of the photosensitive material in creating the interference pattern or diffraction grating at each pixel location. This is time consuming and must be followed by a separate process to remove the unpolymerized portions of the material.
Still another process for creating diffractive microstructures is to use Space Light Modulators (SLM) or Micro-Mirror Devices illuminated with an expanded laser. In such a process, digital representations of line spacings are loaded into the devices and then minified by an optical system. By shrinking the spacings sufficiently, there is the potential to achieve diffraction. In addition, changing a digital file line angle and spacings while keeping minification constant will allow various grating angle and pitch changes, which may produce positional changes to the diffracted beam and a variety of diffractive orders, respectively. A depth of the diffraction grating may be controlled with timed exposure into either light sensitive materials or ablation.
Still another process includes using a laser to ablate a diffraction grating surface to directly form each pixel. Thus, no exposure of photosensitive material is required and no post-exposure treatment is needed to produce the desired pattern. In such a process, an interferometer head splits the laser beam into at least two parts, and then uses a set of angled mirrors to reunite these parts at the surface to ablate the interference pattern on the surface. The azimuthal orientations at which the beam parts reach the surface determine the direction of viewing at which the strongest holographic (rainbow) effect is perceived. To produce different effects at different pixel locations, the azimuthal orientation of the interferometer head relative to the surface has had to be changed intermittently, as well as the angular orientation of the individual mirrors, which form part of the interferometer head. However, due to the mass and inertia of the head, it is difficult to reorient with desired rapidity. Further, any vibrations encountered during reorientation can detract from an extremely high positional accuracy that is desired in order to yield suitable holographic imagery.
One example of a laser ablating system including an interferometer is described in U.S. Pat. No. 6,388,780 to Monaghan et al., incorporated herein by reference in its entirety, and commonly owned with the present application. In Monaghan et al., a pulsed laser beam is directed toward a beam splitter where the beam is split into a first half and a second half. Each beam half is directed along a path to a respective two-axis galvonometer and then to a respective prism. The beams are then directed through a recombiner or condensing lens system and directed to a common focal point on surface, to form a diffraction grating pixel.
In known systems, the surface material to be ablated is a suitable polyimide material. The polyimide ablates in a predictable manner that can be accurately controlled and reproduced through operation of the laser and interferometer to form a predetermined interference pattern or pixel array. That is, in known systems, the laser and interferometer may be operated to control formation properties of the interference pattern and/or pixels, by controlling, for example, depth and location of the diffraction gratings or pixels which form the pattern on the polyimide surface.
However, in the systems above, the holographic effect is multi-chroma, which displays a plurality of colors. For example, gratings formed in a film using the above techniques may reflect the visible spectrum of colors to provide a “rainbow” effect. In some applications, such an effect may be visually displeasing, distracting or otherwise undesirable or unsuitable for a particular application.
Accordingly, it is desirable to provide an optically variable film that may be manufactured as desired to present either multi-chroma or non-chroma visual effects, an apparatus for producing the film and a method of producing the film.